Gravimeter



A. GRAF GRAVIMETER Dec. 3, 196 8 2 Sheets-Sheet 1 Original Filed Jan. 5.1965 W n/r0 Ania/1 GPAF' A. GRAF Dec. 3, 1968 GRAVIMETER 2 Sheets-Sheet2 Original Filed Jan.

wwdat lr r 1 fiwnsvvrm Ania/1 GAAF United States Patent 3,413,854GRAVIMETER Anton Graf, Loehham, near Munich, Germany, assignor toContinental Elektroindustrie A.G. Aslrania-Werke, Berlin-Mariendorf,Germany, a corporation of Germany Continuation of application Ser. No.423,422, Jan. 5, 1965.

This application Aug. 29, 1967, Ser. No. 664,212

Claims priority, application Germany, Jan. 8, 1964,

3 Claims. cl. 73 3s2 ABSTRACT OF THE DISCLOSURE A gravimeter mass ispivoted and restrained for movement in a vertical plane about ahorizontal axis and has an electrically conducting but non-magneticportion located in a permanent magnetic field so that the movements ofthe mass are speed-proportionally damped by eddy currents induced inthat portion. The same mass portion carries a winding energized byreversible direct current from an amplifier which is controlled inresponse to positional departure of the mass from a given null referenceposition. The amplifier output also furnishes a measure of the change ingravity acting upon the mass.

This is a continuation of my copending application Ser. No. 423,422,filed Jan. 5, 1965, for Gravimeter, claiming a right of priority basedupon German application G 39,562 filed abroad Jan. 8, 1964.

This invention relates to gravimeters for measuring the force of gravityon rocking or similarly moving objects, for instance on boats, airplanesor satellites.

Gravimeters are in general highly sensitive spring balances employing aspring system by which an indicator body or gravimeter mass is flexiblymounted. In elementary constructions, when the force of gravity changes,the body deflects by a small amount with respect to the base or support,the deflection serving to measure the change of the force of gravity.The relative motion is measured either visually or, by use of anappropriate motion transducer, electrically.

In measuring the force of gravity with a gravimeter located so that thebase or support itself is in motion, any substantial acceleration of thesite is superimposed on the gravity change in the relative deflection,due to inertia of the indicator. Such acceleration is frequently more orless periodic and sometimes has values corresponding to very largemultiples of the change in the force of gravity sought to be measured.

Measuring under such conditions has been heretofore carried out bymechanically damping the indication weight in such a manner that itundergoes only an amplitude of relative motion of a few millimeters andmeasuring the relative motion by means of an electrical transducer, witha high time constant circuit further aiding in discriminating againstthe spurious deflections caused by acceleration of the base, as in US.Patent 3,019,655 of the present inventor. Assuming the mechanicaldamping attenuation in the gravimeter is 1:150 and that of a voltmeterused for the electrical measurement is 1:200, the total dampingattenuation is 1:30000. Therefore, a periodic oscillation with a maximumacceleration of :150 gal. (1 gal.=1 cm./s. is recorded by the recordinginstrument with an amplitude of only i5 milligal.

This method has the disadvantages that it is period-dependent (longperiods receive less damping than short ones) and causes very longstabilizing times of the measured values. In the above-mentionedexample, it is desirab e to allow to minutes for reaching ofequilibrium. If the boat or other moving support changes 'ice its courseor its speed in a manner somehow producing a transient non-oscillatoryspurious vertical acceleration of the gravimeter mass, it is thusnecessary to wait almost an hour until a measurement can be made.

Similarly, a considerable space interval may occur between the place ofa gravity change and the place of readability of the recording,especially when the measurements are made in airplanes or otherfast-moving vehicles.

In order to minimize this disadvantage, an arrangement has beendeveloped (disclosed in the copending application of Schulze et al.,filed July 26, 1962, Ser. No. 212,516 now abandoned, and theircontinuation-in-part application filed Aug. 23, 1965, Ser. No. 483,022)which constantly returns the indicator to its zero or null position bymeans of a servomotor and a very yielding or sensitive spring. Thereading of the change of gravity is then taken from the state ofexpansion or contraction of the spring. This arrangement has theadvantage of reducing the above-mentioned phase or time lag by reducingthe amplitude of motion of the weight; however, it is found that thisarrangement does not fully solve the problem since the weight stillmoves appreciably from its zero position to actuate the servomotor andthus the long settling-down time is not fully eliminated. Furthermore,this remaining motion can cause a cross-coupling effect when thevertical and the horizontal craft accelerations have the same frequencyand are in a given phase relation to each other.

In accordance with the teachings of the present inven tion, there isemployed in a gravimeter of the general type described above aconstruction in which the movable mass or weight is, upon deflection,urged toward its zero or null position by an electrically controlledforce field (i.e., a force acting through the non-contacting agency of afield), without any mechanical elements introducing delay or inertiaeffects in the application of the compensating force, the current (orvoltage) producing the field which is needed to hold the mass serving atthe same time to measure the change of the force of gravity, thuseliminating the necessity of any mechanical drive and essentiallyeliminating all motion of the weight. The fields which may practicallybe employed are, of course, electrically produced and includeelectromagnetic and electrostatic fields. The invention may accordinglyalso be described as a construction in which an electrical quantity,produced as a function of the position of the gravimeter mass andvariable with the position of the mass, electrically generates a forcefield which produces directlywithout adjustment of a spring or othermechanical dcvicea force which acts against the motion of the mass andholds it substantially in the fixed position, so that the electricalquantity is a measure of the forces acting on the mass.

The basic teachings of the invention, together with further structureand improvements taught thereby, will best be understood from theembodiments of the invention illustrated in the accompanying drawing, inwhich:

FIGURES 1 and 2 are more or less diagrammatic representations ofgravimeter structures made in accordance with the invention.

In FIGURE 1, the numeral 1 indicates a gravimeter mass or body which ismounted on a horizontal axis of rotation by means of torsion springs 2and 3. As described in the above-mentionedpatent of the presentinventor, the mass 1 is disposed in a vertical plane constrained bywires a through 60d, secured at 61, 62, 63, 64, so that it can move onlyin its plane under the effect of accelerative forces. On its outer endis a plate 4, of electrically conductive nonmagnetic material. Thisplate has an extension 5. The plate 4 is movably mounted in the air gapbetween opposed permanent magnets 6 and 7, which cooperate with theplate 4 to produce speed-proportional electromagnetic eddy-currentdamping for the movements of the bod 1.

The extension 5 of the mass or body is rigidly connected with themovably arranged iron rod or core 8 of a ditferential transformer 9. Theiron rod 8 controls the amplitude of the alternating voltage output fromthe transformer 9 to output lines 10. The voltage has a fixed frequency,for instance 100 kilocycles. The alternating voltage disappears at agiven position of the iron core 8 and thus "at a given position of thegravimeter mass 1, namely at the zero or null reference position. Whenthe iron rod 8 passes through this position the phase angle of the ouputvoltage of the transformer 9 changes by 180.

The alternating voltage of the line 10 is amplified in an amplifier 11and phase-sensitively rectified at the output. A direct current flowsthrough the lines 12 to the winding of a coil 13 on the mass between themagnets 6 and 7. The direct current at 12 thus disappears in the zeroposition of the scale balance 1 and reverses its direction of flow whenthe scale balance passes through this position.

The polarity of the current is such that the force produced by thestationary magnets and the magnet formed by coil 13 acts opposite thedirection of deflection. Voltage drops across the resistors R and R arerecorded either in analog form by means of a millivolt recorder 50 whichis damped by a long time constant circuit RC or digitally by means of anintegrating digital voltmeter 51 of relatively low damping (shown as Cwith a printout 52, or both, as illustrated. These recording devices ofcourse provide a continuous gravity profile, as described, for example,in the previously mentioned copending applications.

Since the digital voltmeter illustrated can record only positive ornegative measured values, the voltage base is shifted in this circut bymeans of a stable auxiliary direct voltage 53 so that the variations involtage here do not cross the zero line.

With proper polarity and amplification the weight or mass does notdeflect substantially, but is essentially locked, any mechanical forcewhich tends to deflect it being enhanced by the opposing electromagneticforce. As in all degenerative feedback controls, a very slightdeflection Will inherently occur, of course. However, this deflection iscompletely negligible as compared with the excursion which occurswithout this control arrangement and the change in force exerted by thesprings 2 and 3 is completely negligible. This balance-locking methodaccordingly requires little magnetic damping, because there can be nooscillation requiring damping. Therefore, the time for reachingequilibrium of the measuring element can be kept extremely short, of theorder of only a few seconds, even though large-amplitude oscillations ofthe base at even relatively low frequency are suppressed in theindication. Another advantage is the complete absence of any torqueproduced by horizontal accelerations, which can only occur when the masscan deviate substantially from its zero position. Moreover, a gravimetercontrolled in the present manner is completely linear in calibration toa much greater extent than an instrument whose mass deflectssubstantially for a measurement. Due to the complete linearity of thegravimeter controlled in the manner described, the useful measuringrange is also considerably increased. While formerly a linearcalibration of 1100 gal. could be reached only with difliculty, thepresent instrument can cover i400 gal. or more, depending on thehardness of the control '(i.e., the feedback loop gain). It may beobserved that calibration testing of the present gravimeter does notrequire such complexities as an expensive sine lift (a mechanicalarrangement for the generation of vertical, periodic accelerations ofvarious amplitudes and frequencies). A Cardanic suspension of thegravimeter on a spiral spring with the gravimeter oscillating at itsnatural period of oscillation is suflicient for the purpose. Forinstance, at a period of 2 seconds a periodic acceleration of :400 gal.is attained at an amplitude of only :40 cm. A lift more than 20 metershigh would be necessary for a similar test with conventional dampedconstructions. The periods of oscillatory motion of a boat are around 10seconds, and since ordinary gravi-meters are frequency-dependent withregard to the damping attenuation, the test would have to be carried outwith a period of 10 seconds instead of at 1 or 2 seconds, which ispermissible when the measuring balance is locked in accordance with theinvention. Furthermore. the testing of the gravimeter by means of freeoscillation of a spring is completely shockless, while mechanicallyoperated sine lifts run relatively roughly as a result of the inevitablefriction.

FIGURE 2 shows a similar arrangement to that of FIGURE 1, as regards thebroader teachings of the invention. However, the position transducer 9of the arrangement of FIGURE 1 is here replaced by a photoelectricposition transducer. The extension 5 has an aperture diaphragm 20 whichdistributes a light beam, generated by a 'bulb 21 through an imaginglens 22, between photocells 23 and 24. The photocells 23 and 24 arearranged With their output voltages in a differential circuit so thatwith symmetrical exposure of the two cells the resulting output voltageis zero. Upon unsymmetrical exposure to light, the differential voltagechanges in a positive or negative direction depending on the directionof movement of the diaphragm 20. The direct voltage is amplified bymeans of the amplifier 11A (with appropriate polarity provision) andused as earlier described. The voltage drop across a resistor 25 isagain measured or recorded by means of an appropriate measuring device26.

Many further variations will readily be devised by persons skilled inthe art, both similar and dissimilar to the embodiments described above.Accordingly, the scope of the protection to be given the inventionshould not be determined on the basis of similarity or dissimilarity tothe particular embdirnents here shown, but on the basis of thestructures of the invention as defined in the annexed claims.

In the claims:

1. A gravimeter for use on travelling vehicles, comprising supportstructure; a body constituting a gravimeter mass and having a leverportion pivotally mounted for rotatable movement relative to saidstructure about a horizontal axis; a group of filamentary constrainingmembers of which each has one end attached to said body and the otherend attached to said structure and which are arranged to jointlyconstrain said body to movements in a vertical plane; spring meansdisposed between said body and said structure for elastically biasingsaid body to a substantially horizontal null reference position in saidplane; a controllable force-field device comprising magnet means ofconstant magnetomotive force mounted on said structure and having afield through which said body extends, said body having in said field aportion formed of non-magnetic and electrically conducting material foreddy-current damping of movements of said body, an excitation windingmounted on said body portion in said field; a position-responsiveelectric sensing device for providing an electrical measuring signalindicative of the positional departure of said body from said nullreference position; an amplifier network having an input connected tosaid sensing device and having a direct-current output reversible inresponse to passage of said body through said null reference position,said output being connected to said winding for controlling saidforce-field device to counteract said positional departures; andelectric measuring means connected and responsive to said output tofurnish data indicative of changes in gravity acting upon said body.

2. In a gravimeter according to claim 1, said magnet means comprisingtwo permanent magnets of U-configuration located on horizontallyopposite sides of said body, each of said two magnets having its twopole faces located in a vertical plane one vertically above the other,whereby said two magnets form between each other two field gaps, saidbody portion being flat and extending substantially in a vertical planethrough said two gaps, and said winding on said flat portion havingrespective perimetrically sequential portions extending through said twogaps.

3. A gravimeter comprising support structure; a body constituting agravimeter mass and being pivotally mounted for pendulous movementrelative to said structure about a horizontal axis; constraining meansfor restricting said body to movements in a vertical plane; spring meansdisposed between said body and said structure for elastically biasingsaid body to a substantially horizontal reference position in saidplane; magnetic field means of constant magnetomotive force mounted onsaid structure, said body having a plate-shaped portion extending in avertical plane through the field of said field means and being formed ofnon-magnetic and electrically conductive material for providing eddycurrent damping substantially proportional to the speed of positioneddeparture of said body from said reference position; an excitationWinding mounted on said plate-shaped body portion and extendingsubstantially parallel to said vertical plane and through said field; aposition-responsive electric sensing device for providing an electricalmeasuring signal indicative of the positional departure of said bodyfrom said reference position; an amplifier network having an inputconnected to said sensing device and having an output reversible inresponse to passage of said body through said reference position andconnected to said winding for exciting it to counteract said positionaldepartures; and electric measuring means connected and responsive tosaid output to furnish data indicative of changes in gravity acting uponsaid body.

References Cited UNITED STATES PATENTS 2,316,915 4/1943 Truman 73-3822,367,126 1/1945 James 73-382 2,786,101 2/1957 Kinkel 73-398 3,066,25511/1962 Westphal 73-382 X 2,136,219 11/1938 Scherbatskoy 73-3822,304,324 12/ 1942 Williams 73-382 2,853,287 9/1958 Draper et a1 732,923,904 2/ 1960 Hieber.

3,019,655 2/1962 Graf 73-382 3,176,521 4/1965 Clark 73-517 3,211,00310/1965 Worden 73-382 3,323,372 6/1967 Kistler et al 73-517 JAMES J.GILL, Primary Examiner.

